In recent years, a urea selective catalytic reduction (SCR) system has been introduced as a technique for purifying nitrogen oxide (NOx) contained in an exhaust gas of a work machine, etc. The urea SCR system utilizes a chemical reaction of ammonia with nitrogen oxide to reduce the ammonia to nitrogen and water. For the safety, not ammonia but urea water is stored in a tank and mounted on the work machine or the like with the tank, and the urea water is injected into an exhaust gas having a temperature of approximately 200°. The injected urea water is hydrolyzed at a high temperature, whereby ammonia gas is generated. The chemical reaction of ammonia gas and nitrogen oxide reduces the nitrogen oxide, thereby generating nitrogen gas and water vapor.
For example, Japanese Patent No. 4928304 discloses an exhaust-gas posttreatment apparatus including a gas collection chamber, a mixing pipe having a rear end face closed by a downstream-side end portion of the gas collection chamber, and an injector for urea water addition, the injector concentrically attached to the rear end face. The injector for urea water addition supplies urea water to the center position of rear end portion of the mixing pipe. This addition requires measures to suppress heat transfer from the mixing pipe to the injector for urea water addition because the temperature of the mixing pipe is generally raised by contact thereof with an exhaust gas.
Japanese Patent No. 5349715 discloses an apparatus including a heat insulation section for suppression of the heat transfer. The apparatus includes an injector for supply of reducing agent, an elbow pipe including an injector retention section, and a gasket interposed between the injector and the injector retention section. The gasket includes an inner plate in contact with a retaining wall of the injector retention section and having an inner opening corresponding to an injection opening, an outer plate in contact with the injector and having an outer opening corresponding to the injection opening, an annular outer-circumference-side support ring held between respective outer circumferential portions of the inner plate and the outer plate, and an annular inner-circumference-side heat insulating ring held between a circumferential edge section of the inner opening of the inner plate and a circumferential edge portion of the outer opening of the outer plate. Inside the gasket is formed an intra-gasket heat insulation space covered with the inner plate, the outer plate, and the outer circumference side and inner circumference side rings.
The intra-gasket heat insulation space, though having a heat insulating function, is completely closed, which causes heat accumulation in the intra-gasket heat insulation space; this may rather deteriorate cooling efficiency, i.e., heat insulation efficiency.